The SDXC isn't a change in addressable areas, but a change in the size of the blocks transferred. Electronically, they're compatible. The issue is if the software (drivers mainly) is capable of adjusting it handle the extra data. Linux has had support for over-sized blocks (>512 bytes) for a long time.

Now the new fat format MS cooked up to put fat on these is another thing entirely. There is marginal support for that, both in Windows and linux. So that may be an issue. But if you want to slam in a 128G card and format it to efs3, the N800 should be able to handle that.

If you search the forums, you'll find a couple of posts from someone who already put a 64G SDXC card in their N900 and it worked fine. My understanding is that that part of the sub-system (driver wise) isn't that much different, and is one of the open components in any case. (SD tech isn't exactly trade secret stuff...)

Voltage wise, there's no way to damage a card from trying it. Just the data on it may be corrupted... so try it with a blank one.